1. Field of the Invention
The present invention relates generally to a cross polarization interference eliminating apparatus and a bit shift method for eliminating cross polarization interference. More particularly, the invention is directed to such apparatus and bit-shift method for eliminating cross polarization interference, which are useful in a digital radio multiplex transmitting system.
2. Description of the Related Art
In recent years, for the advantage of the high efficiency of frequency use and bi-directional transmission of large amount data, an increasing number of cross polarization transmission systems have been put on the market as compared to mono-polarization transmission systems, particularly in the field of a digital multiplex radio transmission using microwaves or submillimeter waves.
The cross polarization transmitting system is a kind of radio transmission system for transmitting data using two different polarized waves as carrier waves of a common frequency. The two polarized waves are a vertical polarized plain wave and a horizontal polarized plain wave (hereinafter called the V-polarized wave and the H-polarized wave) as described below.
First, an object signal to be transmitted is modulated in a transmitting apparatus, divided into a V-polarized wave and an H-polarized wave, whereupon they are transmitted in the form of signal-1 on the V-polarized wave and signal-2 on the H-polarized wave. Namely, signal-1 is transmitted by the V-polarized wave and signal-2 is transmitted by the H-polarized wave whose frequency is identical with the frequency of the V-polarized wave. The orthogonalized V-polarized wave and H-polarized wave interfere each other when they are transmitted via radio space. Therefore, a receiving apparatus has to eliminate the interference on both the two polarized waves for preventing any loss of original data.
An apparatus for eliminating the interference is called a cross polarization interference eliminating apparatus or cross polarization compensating apparatus. This apparatus is designated in an effect to make the optimum adjustment as by shifting the sampling timing of the H-polarized wave for eliminating cross polarization interference as much as possible if interference elements from the H-polarized wave is superimposed on the V-polarized wave.
Namely, the V-polarized wave and the H-polarized wave are received in a receiving apparatus and down-converted to be inputted to cross polarization wave interference eliminating apparatuses, where the V-polarized wave and the H-polarized wave are analog-to-digital converted into and demodulated as respective desired baseband signals. The demodulated output signals are equalized in deterioration due to fading or the like in a transversal equalizer, and added in the adding section installed in a subsequent stage where they are converted into desired signal-1 and signal-2, respectively.
FIG. 14 of the accompanying drawings shows the manner in which cross polarization interference-component is eliminated from one of the received polarized plain waves. In the polarization transmitting system 86 of FIG. 14, the transmitted V-polarized wave and H-polarized wave separately propagate through radio space and separately received in a receiving apparatus 84. Interference is developed in antennas on a transmitting apparatus (not shown) and those on receiving apparatuses 90a, 90b and the radio space, and results in a major cause fault-receiving.
The receiving apparatus 84 of FIG. 14 is composed of a signal demodulating section 91 and an interference eliminating section 92. The V-polarized wave is received in the antenna 90a, down-converted in a mixer 82a, and analog-to-digital converted in an analog-to-digital converter 91b, whereupon the digital signal is inputted to a first demodulator 91c. The demodulated digital signal is equalized in a transversal equalizer 91d, added in an adding section 91f to the demodulated signal of the other polarization, and inputted to an error detecting section 93 to output a detection signal.
On the other hand, the H-polarized wave is received in the antenna 90b, and the analog signal of the H-polarized wave is delayed by a delay element 83 for adjustment to conform the signal time to the V-polarized wave. Then the H-polarized wave is analog-to-digital converted in an analog-to-digital converter 92b and the digital signal is inputted to a second demodulator 92c. The demodulated signal is equalized in a transversal equalizer 92d, and is inputted into the adding section 91f where the resulting signal is added to the demodulated baseband signal to eliminate the interference-component.
The equalizations of the transversal equalizers 91d, 92d are signal processing using delay taps constituting the transversal equalizers 91d, 92d. The transversal equalizers 91d, 92d include a plurality of taps, which are flip-flops connected in series, shifting the bit time of the input signal. Accordingly, the transversal equalizers 91d, 92d equalize and eliminate multipath-components by correlation arithmetic of shifted digital signals on the delay taps, respectively. The amount of interference is extracted in the transversal equalizers 91d, 92d and is input to controllers (CONTROL) 91e, 92e as interference-component signals to ensure an error-free interference eliminating operation. In addition, the amount of interference is utilized to control the operation speed of the analog-to-digital converter 91b, 92b and the delay amount of the delay element 83.
The delay element 83 delays an analog signal by a predetermined time, which is controlled by the controller 92e and is set so that the sampling is carried out at the optimum timing. At the final stage, the error detector 93 outputs a canceling signal. Then the error signal outputted from the error detector 93 is inputted to the transversal equalizer 92d to change the tap coefficients. The oscillator 91n inputs the converting clocks of the analog-to-digital converters 91b, 92b, being controlled by the controller 91e. 
However, according to the receiving apparatus 84, unless the receiving wave signal input to the demodulating section 91 and other polarized wave signal input to the interference eliminating section 92 are inputted simultaneously, the interference-component cannot be eliminated sufficiently, and an accurate correlation value cannot be obtained to cancel the amount of interference, resulting in inadequate performance of the polarization interference elimination.
Further, the delay element 83 cannot change the amount of delay without cell breathing; with the setting being fixed, the delay element 83 cannot make a change of the delay amount while the system is working.
Furthermore, the delay element 83, being an analog element, can change the amount of delay due to change of environment, such as temperature, and would be affected by on which signals are transmitted. Therefore, the delay element 83 should be set with taking the foregoing influences into consideration. Setting the amount of delay requires manipulation by an operator watching a monitor on which target values are displayed. Consequently, new technology has been cherished which realizes a polarization interference eliminating apparatus that enables an error-free adjustment, is compact in size, and is stabilized in operation.
With the foregoing problems in view, it is an object of the present invention to provide apparatus and bit-shift method for eliminating cross polarization interference in which an amount of delay is changed without cell breathing irrespective to an operator and which operates stably without an adjustment and which is constituted to be high-density.
Therefore, according to the first aspect of the present invention, there is a provided the cross polarization interference eliminating apparatus comprising a signal demodulating section for receiving a first signal resulting from one of radio signals, which are transmitted in a modulated and encoded form respectively using two kinds of polarized plane waves intercrossing at right angles in a common frequency band, and for analog-to-digital converting, demodulating and equalizing the received first signal to output a first baseband signal; an interference eliminating section for receiving a second signal resulting from the other radio signal, analog-to-digital converting the received second signal to obtain a digital signal, delaying the obtained digital signal by a predetermined time, and demodulating and equalizing the resulting digital signal to output a second baseband signal; and an adding section for adding the second baseband signal from the interference eliminating section to the first baseband signal from the signal demodulating section to output a composite signal; the interference eliminating section including: an analog-to-digital converter for converting the second signal to output a digital signal; a demodulator, connected to the analog-to-digital converter, for delaying the digital signal from the analog-to-digital converter by the predetermined time; an equalizer, connected to the demodulator, for equalizing the digital signal from the demodulator, the equalizer having a plurality of taps whose coefficients are variable for outputting the second baseband signal; a controller, connected to the equalizer, for reading/writing the coefficients of the taps in the equalizer and outputting a control signal to the outside of the controller, and a timing adjuster, connected to the controller and the demodulator, for setting a delay time in the demodulator in accordance with the control signal from the controller.
The above-mentioned apparatus enables to adjust delay time automatically, sample at the optimum timing and control delay of a digital signal to eliminate interference-component sufficiently. And as another advantage of the present invention, if cross polarization does not interfere, interference is eliminated at the normal timing; otherwise if cross polarization interferes, interference is eliminated by generating a predetermined amount of delay.
According to the second aspect of the present invention, the timing adjuster may include a counter, connected to the controller, for outputting n kinds of binary signals, which are synchronized with a high-speed clock equal to n times a clock speed of the demodulated signal, and a low-speed clock, which is synchronized with the clock speed of the demodulated signal, where n is a natural number, and a decoder, connected to the counter and having n output lines, for inputting to the demodulator binary digits of the n output lines in synchronism with the high-speed clock in such a manner that the binary digit of only one of the n output lines is different from the binary digits of the remaining output lines in correspondence to the n kinds of binary signals from the counter; and wherein if the control signal is an activation command, the binary digits of the n output lines in the decoder are updated, and if the control signal is an inactivation command, the binary digits of the n output lines in the decoder are not updated.
Accordingly, such a timing adjuster enables to occur bit-delay of a predetermined time by controlling pulse of a control signal to delay digital signals.
As a preferable feature of the present invention, the timing adjuster may include an output unit for outputting binary digits of n output lines, which are synchronized with a high-speed clock equal to n times a clock speed of the demodulated signal, in such a manner that the binary digit of only one of the n output lines are different from the binary digits of the remaining output lines, where n is a natural number, and a clock generator for outputting a low-speed clock, which is synchronism with a clock speed of the demodulated signal; and wherein if the control signal is an activation command, the binary digits of the n output lines in the output unit are updated, and if the control signal is an activation command, the binary digits of the n output lines in the output unit are not updated.
According to the timing adjuster, the amount of delay or precede enables to be adjusted based on the amount of delay of digital signals. And the amount of interference in the demodulating section is followed within the real time so that the amount of delay is changed without cell breathing.
As another preferable feature of the present invention, the demodulator may have a first holding sections, connected to the analog-to-digital converter and the timing adjuster, for rewriting holding portions corresponding to the n output lines from the timing adjuster into a value of the demodulated signal to output binary digits of n bits; a second holding section, connected to the first holding section and the counter in the timing adjuster, for storing the binary digits of n bits from the first holding section in synchronism with the low-speed clock from the counter; and a third holding section, connected to the second holding section, for storing the binary digits of n bits, which are stored in the second holding section, at a clock speed synchronized with the clock speed of the demodulated signal to be inputted from the outside of the third holding section.
Accordingly, it is possible to change the operation even while the system is working without operation by an operator, fine adjustments can be realized. Also it is possible to set the amount of delay, regardless of the change of environment, such as temperature.
The controller further may have a timing detector for extracting an amount of interference of cross polarization, based on the coefficients of the taps in the equalizer and outputting a value of the extracted amount as amount-of-delay information; and a control-signal output device for outputting the control signal to activate the demodulator to delay, based on the amount-of-delay information from the timing detector and an interference-component signal from the outside of the control-signal output device.
Therefore, digital signal inputted to the demodulating section and digital signal inputted to the cross polarization interference eliminating section are adjusted to synchronize with each other and to be eliminate interference-component easily.
As still another feature of the present invention, a bit-shift method for eliminating interference of cross polarized plain waves in synchronism with a multiplied clock equal to n times a clock of demodulated baseband signal, where n is a natural number, the method comprising the steps of discriminating, based on an amount of the interference of cross polarized plane waves, whether or not a digital signal should be delayed; if the digital signal should not be delayed in the discriminating step, outputting the n kinds of binary signals, which are synchronized with the multiplied clock, and a low-speed clock, which is synchronized with a clock of the demodulated baseband signal, and if the digital signal should be delayed in the discriminating step, stopping the outputting of the n kinds of binary signals and the low-speed clock; designating a particular holding portion in the first holding portion, which has n holding portions each holding a binary digit; writing a binary digit of 1 bit of the digital signal, which is synchronized with the multiplied clock, in the particular holding portion designated in the designating step; writing binary digits of the individual holding portions, which constitute the first holding section, one into each of n holding portions in a second holding section where each holding portion may hold a binary digit, by the low-speed clock; and writing binary digits of the individual holding portions, which constitute the second holding section, one into each of n holding portions in a third holding section where each holding portion may hold a binary digit, by the baseband signal clock.
According to the above-mentioned method, the sampling timing is adjusted automatically by the timing adjuster to eliminate the other polarized plain wave interference component as much as possible. Since the simple digital apparatus controls amount of delay, it is possible to promote introduction of the present system with advantage of reasonable amount of investment for manufacturing, as well as labor-free adjustments, reduction of apparatus to a compact size and stabilized operation.